The present invention relates to circuitry for identifying which subscriber of a two-party telephone line is initiating a toll call and is directed more particularly to circuitry for providing tip automatic identification (tip ANI) for two-party lines.
In telephone systems it is often desirable to establish a two-party line be connecting two different subscribers or parties to the same telephone line. Although many signalling functions are not affected by having both parties connected to the same line, a problem arises when it is necessary for the central office equipment to identify which party of the two-party line is initiating a toll call.
Currently, it is the practice in the industry to connect, to the telephone set of a designated one of the parties (hereinafter referred to as the tip party or party two), circuitry which applies ground potential to the tip conductor through a current-limiting resistor. The subscriber drop of the other or nondesignated party (hereinafter referred to as the ring party or party one) does not have such circuitry connected thereto. As a result, when the central office equipment establishes the toll ticketing condition by applying negative potential to both conductors of the telephone line, a current will flow through the tip conductor only when party two initiates a toll call. The central office equipment detects the presence or absence of a current flow in the tip conductor and thereby identifies which party has initiated the toll call.
If, for example, the ring party (party one) initiates a toll call, no current will flow through the tip conductor when the central office equipment applies negative potential to both conductors of the telephone line. The central office equipment will detect that no current is flowing through the tip conductor and will thereby determine that party one has initiated the toll call. If, on the other hand, the tip party (party two) initiates a toll call, the ground connection to the tip conductor will cause a current to flow through the tip conductor when the central office equipment applies negative potential to both conductors of the line. As a result, the central office equipment will detect the current flow through the tip conductor and will thereby determine that party two has initiated the toll call. Thus, during the toll ticketing condition, the presence or absence of a current flow in the tip conductor of the telephone line enables the central office equipment to identify which party has initiated the toll call.
As described above, the toll ticketing condition is established when the central office equipment applies negative potential to both conductors of the telephone line. In different telephone systems, the central office equipment may establish the toll ticketing condition at different times. In one type of telephone system, the toll ticketing condition is established during dialing. In these telephone systems, loop current flow to one of the subscribers is established when that subscriber goes off-hook. The central office senses the loop current and applies a dial tone to the telephone line to signal that the subscriber may begin dialing. The dial pulses are sensed by the central office equipment in order to operate the switching circuits. In addition, the central office senses the occurrence of the first open interval of a designated digit, e.g., the first open interval of the second digit, and establishes the toll ticketing condition by applying negative potential to both conductors of the telephone line during a portion of this open interval. During this portion of the open interval, the central office senses the absence or presence of current flow in the tip conductor in order to identify which one of the subscribers has initiated the toll call. Upon completion of the toll ticketing condition, the central office equipment returns to the dialing condition. It will be understood that the establishment of the toll ticketing condition during an open interval of a dial pulse does not interfere with the dialing operation.
In other types of telephone systems, the toll ticketing condition may occur before dialing, after dialing, or both before and after dialing. In telephone systems which establish the toll ticketing condition both before and after dialing, loop current flow to one of the subscribers is established when that subscriber goes off-hook. At this time, the central office equipment establishes the toll ticketing condition by applying negative potential to both conductors of the telephone line and senses the absence or presence of a current flow in the tip conductor of the line in order to identify which party has initiated the toll call. After the toll ticketing condition has been terminated, the central office equipment applies a dial tone to the telephone line to signal that the subscriber may begin dialing. The dial pulses are sensed by the central office equipment and operate the switching circuits. After all of the digits have been dialed, the central office equipment again establishes the toll ticketing condition and senses the presence or absence of a current in the tip conductor in order to identify which one of the parties has initiated the toll call.
Many prior art tip ANI circuits comprise a resistor and inductor connected between the tip conductor and ground whenever the subscriber is off-hook. This ground connection to the tip conductor is maintained during the calling and talking states. As a result, these resistor-inductor ANI circuits allow an undesirable electrical noise to appear in the telephone line. In addition, these prior art ANI circuits have been physically housed in the telephone set. As a result, false billings occur when unauthorized phones not having these resistor-inductor ANI circuits are exchanged for or are added to existing phones having that type of ANI circuit. Thus, many prior art ANI circuits are undesirable because they introduce noise into the telephone line and may cause false billings.
Realizing the problems associated with the above-described resistor-inductor type ANI circuits, a few ANI circuits have been designed which do not connect ground to the telephone line during the talking state and which are to be connected to the outside of the building housing the subscriber set. While these circuits do not introduce excessive noise in the telephone line and allow the subscriber to expand or exchange phones without having the problem of false billings, they are too complicated and, therefore, too expensive to be practical.